define

    name sub attribute,
      datatype string;
    started-at sub attribute,
      datatype date;
    duration sub attribute,
      datatype long;
    first-name sub attribute,
      datatype string;
    last-name sub attribute,
      datatype string;
    phone-number sub attribute,
      datatype string;
    city sub attribute,
      datatype string;
    age sub attribute,
      datatype long;
    is-customer sub attribute,
      datatype boolean;

    contract sub relation,
        relates provider,
        relates customer;

    call sub relation,
        relates caller,
        relates callee,
        has started-at,
        has duration;

    company sub entity,
        plays provider,
        has name;

    person sub entity,
        plays customer,
        plays caller,
        plays callee,
        has first-name,
        has last-name,
        has phone-number,
        has city,
        has age,
        has is-customer;
